Project Summary/Abstract Suicide is a leading cause of death in the US, and its rate continues to increase. Most individuals who die by suicide are in contact with health care, but clinical risk assessment is challenging. Inflammatory biomarkers have tentatively been linked to suicide. However, longitudinal studies establishing their accuracy in tracking suicidal behavior and critical symptoms are lacking. Therefore, we propose a longitudinal study with 1,280 assessments, measuring suicidal behavior, associated clinical symptoms and blood biomarkers of inflammation. We will also analyze the inflammatory mediators in postmortem brain tissue from suicide decedents. Our overriding aim is to identify a set of biomarkers that distinguish patients with suicidal behavior from depressive patients without suicidal behavior. Further, we intend to define biomarkers that are elevated during active suicidal behavior (at- risk periods) within the same patients. Our working model is that inflammation (via pro-inflammatory cytokines) induces the kynurenine pathway, leading to an increased production of neurotoxic kynurenine metabolites (i.e., the NMDA-receptor agonist quinolinic acid), which triggers suicidal behavior. We predict that the elevated quinolinic acid in suicidal individuals is associated with epigenetic changes, regulating the expression of kynurenine enzymes in blood and brain cells. We hypothesize that inflammatory cytokines and kynurenine metabolites in plasma are biomarkers of suicidal behavior, and that similar changes will also be evident in postmortem brain tissue from suicide decedents. To test this, we will pursue three specific aims: (1) Establish biomarkers that indicate risk for active suicidal behavior; (2) Quantify levels of inflammatory mediators in postmortem brain tissue from suicide decedents; (3) Determine epigenetic marks in blood and brain tissue of patients with suicidal behavior. In Aim 1, we will enroll patients with Major Depressive Disorder (MDD) and active suicidal behavior, and MDD patients without current or past suicidal behavior. Each subject will be assessed at eight time-points over one year, including two assessments at admission and discharge at an inpatient ward. We will measure interleukins and acute phase reactants as well as tryptophan, serotonin and metabolites of the kynurenine pathway in peripheral blood. In Aim 2, we will measure the inflammatory biomarkers in post-mortem brain tissue from medication-free suicide decedents from the same diagnostic groups and controls. We expect that suicide will be associated with inflammation in brain and increased levels of key kynurenine metabolites, reflecting an altered epigenetic regulation of enzymes in the pathway. Last, we will perform whole-genome methylation analysis using Illumina EPIC arrays, followed by gene pathway analyses, both in blood of the enrolled patients and in postmortem brain tissue. Our project will aid the implementation of biomarkers in clinical care for patients with suicide risk, in order to enable intensified intervention during critical time-points. The biological insight obtained here can guide therapeutic development specifically targeting suicidality, with the ultimate goal of reducing suicide numbers.